1. Field of the Invention
The invention is an apparatus for the capture of sediment in water entering a manhole for an underground storage tank. The invention also is an apparatus providing secondary containment for spills of liquid from within a sump of an underground storage tank.
2. Description of the Related Art
The problem addressed by the Invention is the flooding of electrical pumps and other subterranean systems serving underground storage tanks due to the blinding of underground water drainage systems by sediment and debris.
Underground storage tanks (“USTs”) are tanks designed to contain liquids such as petroleum products and are buried below the surface of the ground. A typical UST used in a gasoline filling station may contain 2,000 gallons or more of fuel. In a typical UST installation, the top of the tank is located thirty inches or more below a concrete driving surface. The UST is surrounded by sand, gravel or crushed stone, which provides even support to the tank and acts as a drainage medium to allow water to drain away from the area of the tank. As used in this application, the term ‘drainage medium’ includes crushed stone, sand, gravel and any equivalent material.
A manway communicates between the surface of the ground and a tank sump at the top of the UST. The tank sump generally is cylindrical in shape and is attached to the top of the tank. A typical tank sump is defined by an open-top, cylindrical perimeter wall that forms a water-tight and liquid-tight connection with the top of the UST. The interior of the tank sump provides access to the tank for adding or removing liquid from the tank, to allow makeup air to enter the tank, or for other access to the interior of the tank. The tank sump may include an electrically operated pump, associated piping and sensors. The pump and piping are configured to withdraw liquid from the bottom of the tank and to discharge the liquid through piping to liquid dispensers. A single UST may incorporate more than one tank sump and be served by more than one manway.
The tank sump perimeter wall serves to contain liquid, such as a petroleum product, that may be spilled or that may leak from the systems serving the UST and that are located within the tank sump. Prevention of release of the liquid into the environment is important for environmental protection and for regulatory compliance. If the liquid escapes from the tank sump, as by overflowing the tank sump perimeter wall, the liquid is released into the environment, causing environmental damage and causing the owner of the UST to incur remediation costs.
A circular manhole skirt having a diameter greater than the tank sump defines a manhole, which is the entrance to the manway. A manhole cover is supported by the manhole skirt and covers the manway. The top of the manhole skirt and manhole cover generally are located at grade so that the manhole skirt and manhole cover do not present obstacles to travel; for example, so that the manhole skirt and cover do not prevent movement of vehicles on pavement located above the UST. As used in this application the term ‘grade’ means the elevation of the surface of the ground, concrete driving surface or other material below which the underground storage tank is buried.
The tank sump and lower portion of the manhole skirt are annularly arrayed, with the outer diameter of the tank sump perimeter wall being less than the inner diameter of the manhole skirt side wall. The area between the manhole skirt side wall and the tank sump perimeter wall defines an annular space communicating between the manway and the drainage medium surrounding the UST.
In a typical prior art UST installation, water from surface runoff can and does enter the manhole past the manhole cover. The source of the water may be rain, snow, or any other source of water. The water enters the manway at the periphery of the manhole skirt and runs down the manhole skirt side wall. The water encounters the crushed stone surrounding the tank when the water reaches the bottom of the manway. The water drains through the drainage medium and away from the tank.
When surface runoff enters the manway, the water carries with it entrained sediment and debris. When the water flows through the drainage medium, the sediment and debris are left behind in the drainage medium. Over a period of years, the sediment and debris accumulate in the drainage medium, filling the spaces between the individual stones and preventing water from draining through the drainage medium. Over time, the drainage medium becomes blinded by the sediment and debris and ceases to function. The failure of this drainage system allows the tank sump to flood, damaging tank systems such as electrical pumps, sensors, valves, piping or related systems.